"Overbore: A loose term used to describe a case that has more capacity than it can effectively use with normally available powders. See also: Bore Capacity and Expansion Ratio." --Sierra Rifle Reloading Manual, 4th Edition
Any given barrel can only have X pounds of powder run through it. It is usually prudent to maximize the efficiency of powder burned per projectile.

Would this be the same as saying that the case is to big in reference to the caliber size?

Does this have an adverce affect on accuracy? If so, Why?

What I'm having trouble with is this...

Many post here and on other sites say that the 6.5 WSM is overbored. If a 140 AMAX say at 3000 fps shoots accurately from a 6.5WSM and shoots equally well from a 6.5/284 though powder weights vary, and the WSM case has leftover room what does it matter? Why is the 6.5/284 more accurate according to many, a fact that I'm not disputing because I don't own either. In this example, is this one of those "inherently accurate" rule or is it really the case of the WSM being over bored?

If you figure out the amount of powder to reach different velocity levels it will help show why some cartridges are considered more efficient.If we stick to a 107 Sierra bullet in all of the examples it will help.6BR at 2950 fps uses 31 gr of powder.A 243 will take 43.5 gr to get the same velocity.A 244 will take 44.5 gr to get the same velocity.If you Ackley improve the 244 case it takes 1 less grain or 43.5 of powder to get the same velocity as the standard case.If we go to 6/284 it takes 43 gr to get the same velocity while a 6mm-06 will take 46.8 grains as does a 240 Weatherby.In this example with the velocity limited the little 6BR is getting 95 feet per second per grain of powder burned.The 6mm-06 is getting 63 feet per second per gr of powder burned and is therefore less efficient.The same thing can be done with the 6.5X284 and the 6.5 WSSM to show which one is more efficient but as i don't own either can't give you the actual numbers.If you compare some of the big 30 caliber rounds with lighter bullets the term overbore is easier to see.A 300 Weatherby needs 78 gr of powder to push a 165 gr bullet 3300 fps.The 300 Ultra mag needs approximately 15 gr more powder depending on type to do the same thing and the 30-378 needs up to 118 gr to get the same velocity.That is 40 grains more powder or a whole 308 winchester load added on to get the same velocity.I would consider the 30-378 overbore for shooting 165 grain bullets and not very efficient with that weight bullet.
Lynn

Lynn put it pretty well. Bigger cases, more powder(all else being equal). More powder to do the same thing means more recoil(which is bad for accuracy), and shorter barrel life. It also means more powder burn inconsistancies due to temps or inherent tolerances(kernal variations, case capacity differences,etc). +/- %5 of 50gr is better than the same variance @ 80gr.
Precision shooters(Benchrest), achieve consistancy using the least amount of powder and capacity for a caliber.
Accurate shooters(Varmint), turn to larger capacity wildcats to reduce drop % drift. They expend alot of effort with ammo & barrels.
Tactical shooters(.308 crowd), more or less, split the extremes. They shoot plenty well, for a long time. They have more fun, really.

"Would this be the same as saying that the case is to big in reference to the caliber size?

"Does this have an adverce affect on accuracy? If so, Why?

"What I'm having trouble with is this...

"Many post here and on other sites say that the 6.5 WSM is overbored. If a 140 AMAX say at 3000 fps shoots accurately from a 6.5WSM and shoots equally well from a 6.5/284 though powder weights vary, and the WSM case has leftover room what does it matter? Why is the 6.5/284 more accurate according to many, a fact that I'm not disputing because I don't own either. In this example, is this one of those "inherently accurate" rule or is it really the case of the WSM being over bored?"

MAELTY
Yes, among other things "overbore" means that the ratio of the case to the caliber is too big to maximize efficency.
Accuracy depends on so many variables it is difficult to say if overbore is a factor. As Mikecr notes, the short range benchrest guys seem to maximize accuracy with small cases launching 6mms. However, both one mile and 1000 yard benchrest accuracy records seem to be set with overbore cases.
It is notable that the .243 Win, which is overbore seems to be inherently more accurate than the .308 Win -- same case different caliber. In terms of the 6.5, I have lots of experience with the 264 Winmag, the 6.5/06 the 6.5/284 Norma and the 6.5x55 Swede. All except the 6.5 Swede are overbore at different levels with the 264 Winmag being the most overbore. All are accurate in my experience with the 6.5/284 being the top performer both at 100 and 1000. The 6.5 WSM or its variants may be as accurate as the 6.5/284 but I doubt that the former is as efficient as the latter in terms of barrel life. The most accurate 100 yard rifle I've ever owned was a 220 swift which held five in 1/4 inch after the throat was almost completely eroded by 2,000 plus rounds. That said, the cone of dispersion did not hold accuracy much beyond 300 and at 500 it patterned like birdshot.
There are alot of super overbore rounds out there but most of them are marketing gimmicks in my opinion.

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